1. Field of the Invention
This invention relates to radio communication systems of a multichannel access type which detect an unoccupied one of a plurality of radio channels to transmit/receive voice and data and, more particularly, to radio communication systems which ensure a quantity of traffic by using a plurality of control channels.
2. Description of the Related Art
FIG. 7 is a block diagram of a conventional structure of a radio communication system of this type. In FIG. 7, a base unit 1 is connected to a radio telephone 2 via a radio circuit and also to a wire-telephone line 3.
A signal incoming from the telephone line 3 is input via a hybrid circuit 4 to a transmitting unit 5 to modulate a carrier wave therein. The signal modulated by the transmitting unit 5 is transmitted via an antenna duplexer 6 from an antenna 7 to a radio telephone 2.
In the base unit, the electric waves transmitted by the radio telephone 2 are received by a receiving antenna 7 and demodulated by a receiving unit 8 via the antenna duplexer 6. The demodulated signal is transmitted via the hybrid circuit 4 to the wire-telephone line 3.
A synthesizer 9 outputs to the transmitting and receiving units 5 and 8 a signal having a frequency corresponding to radio channel means including a talking channel and a control channel. In that case, the control channel constituting part of the radio channel means is one in number and the talking channel is two or more.
One output from the receiving unit 8 is input to a received-field strength detector sensor 10 to determine the field strength. The detector 10 is generally referred to as a carrier squelch circuit or a noise squelch circuit. Another output comprising a data signal contained in the demodulated received signal is input to an identification signal detector 11 which checks an identification signal determined by a combination of the base unit 1 and the radio telephone 2. The identification signal is generally referred to as an ID code.
The detector outputs and the demodulated data signal are input to a control circuit 12 to be used for the purpose of connection control. Further, the control circuit 12 controls the synthesizer 9 to switch among the radio channels and to cause data to be input to the transmitting unit for modulating purposes.
Similarly, the radio telephone 2 includes a receiving antenna 13, a receiving unit 14 and an antenna duplexer 18. The demodulated output from the receiving unit 14 is input to a receiver 15. Voice input by a transmitter microphone 16 becomes a modulation input to the transmitting unit 17 to modulate the carrier therein and the modulated signal is transmitted via the antenna duplexer 18 from the antenna 13.
A synthesizer 19, a received-field strength detector 20, and an identification signal detector 21 are equivalent to the corresponding ones of the base unit 1. A control circuit 22 controls the whole radio telephone 2. A loudspeaker 23 is a sounder which generates a ringing tone when there is an incoming call. The base unit 1 has an AC plug 24 connected to an AC 100 V power source and the output from a rectification stabilizer 25 is supplied to the respective circuits of the system. The output from the stabilizer 25 is also supplied via a current control resister 26 and charging terminals 27, 28 to a chargeable battery 29 which is used as a power source for the radio telephone 2. In this conventional example, control is performed as schematically shown in a flowchart of FIG. 8 when there is an incoming call. When the base unit 1 detects an incoming call signal from the wire-telephone line 3 using a calling signal detector 30 when the base unit is on standby (step 33), it checks to see if the control channel C-CH is unoccupied (step 34). If so, the oscillating frequency of the synthesizer 9 is set to a control channel frequency, and turns on the transmitting unit 5 to thereby cause the incoming signal to be transmitted (step 35). In this case, the incoming signal contains a signal designating a talking channel S-CH.
The radio telephone 2 turns on the synthesizer 19 for a predetermined time t1 set in a timer (not shown) when it is on standby, sets the oscillating frequency of the synthesizer at the control channel frequency, and turns on the receiving unit 14 (step 36). At this time, when there is an incoming signal received (step 37), the transmitting unit 17 is turned on (step 38), an acknowledge signal is transmitted (step 39), and the designated talking channel S-CH is selected (step 45). If there is no incoming signal received, the synthesizer 19 and receiving unit 17 are turned off for a predetermined time t2 (step 40). Intermittent reception of a signal by turning on and off the receiver 14 is for energy saving purposes.
If the consumption current flowing through the receiving unit which is off is Ioff and the consumption current flowing through the receiving unit which is on is Ion, the average consumption current I flowing when the receiving unit is on standby becomes I=(1/t1+t2).times.(t1.multidot.Ion+t2.multidot.Iof). Since usually Ion&gt;&gt;Ioff, I is reduced.
When the base unit 1 detects the electric waves from the radio telephone 2 by the received-field strength detector 10 (step 41), it stops transmission of the incoming signal (step 42). At this time, if no electric waves from the radio telephone 2 are detected, the incoming signal continue to be transmitted up to a predetermined number of times n (step 43). The reason for this is that the radio telephone 2 receives signals intermittently and cannot receive signals during the time interval t2. The reason why the transmission is terminated after the n-time transmission is to avoid useless occupation of the control channel, for example, when the radio telephone 2 is turned off or when the telephone 2 is located at a position excessively remote from the base unit 1.
When the ID codes coincides (step 44), the S-CH designated by the incoming signal is selected. If the ID codes do not coincide, the base unit waits for the disappearance of the call from the wire-telephone line 3 (step 57) and returns to its standby state because the telephones of other communication devices may respond.
After S-CH is selected, the base unit 1 transmits a bell ringing signal (step 47). When the radio telephone 2 receives this signal (step 48), the sounder 23 sends a ringing tone (step 49). When the switch 31 is manually switched on in response to the ringing tone to thereby perform the off-hook operation (step 50), the telephone 2 sends outs an off-hook signal (step 51) and enables telephone communication (step 55).
On the other hand, when the base unit 1 receives the off-hook signal step 52 , it stops transmission of the bell ringing signal (step 53), closes a line relay 33 to establish a communication loop with the wire-telephone line 3 to enable telephone communication (step 54).
After the number of transmissions has become n, the base unit 1 detects the termination of the incoming signal from the telephone line 3 (step 56) and returns to its standby position to thereby prevent a wasteful repetition of the above operations caused by the incoming signal.
The operation of transmission from the radio telephone 2 is shown in a flowchart of FIG. 9. When the switch 31 is switched on to call the wire-telephone line 3, the control circuit 22 responds to this switching operation and determines that it should cause transmission to be performed (step 57), locks the oscillating frequency of the synthesizer 19 to the frequency of the control channel, and turns on the receiving unit 14 to cause the same to receive electric waves through the control channel C-CH (step 58). The telephone 2 detects the field strength of the electric waves received through the control channel by the signal from the received-field strength detector 20. If the received field effect is not higher than a predetermined value, the radio telephone determines that the control channel is unoccupied (step 59), and turns on the transmitting unit 17 to cause same to transmit the ID code signal allocated to the telephone 2 (step 60). When the base unit 1 receives this ID code signal, it determines whether the ID code coincides with the ID code allocated to the combination of the base unit 1 and radio telephone 2 (steps 61, 62). If so, the base unit 1 turns on the transmitting unit 5 to thereby cause same to transmit to the radio telephone 2 a response signal comprising the ID code and the information designating S-CH) (step 63).
The telephone 2 receives the response signal from the base unit 1 by means of the control channel (step 64), and determines whether the ID code contained in the response signal coincides with the ID code allocated to the telephone 2 (step 65). If so, the telephone 2 switches the oscillating frequency of the synthesizer 19 to the frequency of the talking channel designated by the base unit 1 (step 66). After having transmitted the response signal, the base unit 1 also changes the oscillating frequency of the synthesizer 9 to the frequency of the talking channel designated for communication purposes (step 67). Thus the base unit 1 and telephone 2 are connected to the talking channel designated by the base unit 1. Thereafter, by the dialing operation using a dial key 32, the telephone 2 can call the telephone connected to the wire-telephone line 3 to thereby enable telephone communication.
If the telephone 2 receives no response signal from the base unit 1 a time t3 after it has transmitted the ID code signal because the telephone is excessively distant from the base unit 1 or for some other reason, the control circuit 22 causes the loudspeaker 23 to produce an alarm sound after a lapse of time t3 to thereby inform that the connection is impossible (steps 68, 69).
As described above, the conventional radio communication apparatus checks to see if the single control channel is unoccupied, uses the channel to transmit the ID code signal if the channel is unoccupied, and transmits/receives information on a talking channel used for communication if the channel is unoccupied. The reason why the radio communication apparatus checks to see if the control channel is unoccupied is to prevent interference with another radio telephone, which is being connected, caused by transmitting electric waves.
In order to ensure a large amount of traffic, it is necessary to dispose a plurality of radio communication apparatus, each including such base unit 1 and such radio telephone 2 and to increase the whole number of talking channels.
There is the problem that even if a plurality of radio communication apparatus having the conventional structure is used, it is impossible to ensure a quantity of traffic satisfying an increase in the number of talking channels because the control channel of each apparatus is the same, so that the frequency of use of the control channel increases in proportion to an increase in the talking channels and hence the waiting time taken until an unoccupied state of the control channel is obtained increases. Furthermore, as the waiting time for an unoccupied control channel increases, as soon as the control channel becomes unoccupied, a plurality of radio communication apparatus start to transmit signals simultaneously to thereby cause cross talk among them.
In addition, for example, as shown in FIG. 10, assume that radio communication apparatus are disposed, one including such base unit 1 and such radio telephone 2 and the other including such base unit 1' and such radio telephone 2'. Furthermore assume that when there is an incoming signal from the wire-telephone line 3 to the base unit 1, that the base unit 1' is in telephone communication with radio telephone 2' using frequencies f1 and f1'. respectively, in the talking channels adjacent to the control channel and that the radio telephone 2' is located at a position very close, to the base unit 1. In that case, the electric waves having the transmission frequency f1 from the radio telephone 2' leak to the control channel of the base unit 1 and that the received-field strength detector 10 of the base unit 1 continues to output the signal "there is a received electric-field". In that case, the base unit 1 cannot proceed to a step 33 and subsequent steps of FIG. 8 until the telephone 2' terminates its communication, so that the base unit 1 cannot virtually be used.
FIG. 11 shows that base units shown by reference numerals 1, 1' and 1" and radio telephones shown by reference numerals 2, 2' and 2" are disposed such that the base units 1, 1' and 1" are connected to radio telephone lines 3, 3' and 3", respectively. It is assumed that the telephone 2" in the vicinity of base unit 1 is in telephone communication with the base unit 1" through talking channels having frequencies f2 and f2' adjacent to the control channel and that the radio 2' is in telephone communication with the base unit 1' through talking channels having the frequencies f1 and f1' in the same state as in FIG. 10. In that case, at the base unit 1 which has received an incoming call from the wire-telephone line 3, the mutual modulation 2f1-f2=fc (fc =the frequency of the control channel) is caused by the frequencies f1 and f2 electric waves, which is the same state as when the control channel has received electric waves. Thus the base unit 1 cannot be used until the talk between the telephone 2' and 2" is terminated as in FIG. 10.
This applies to the radio telephones. By the arrival of the leaking electric waves or the mutual modulated electric waves at the control channel, the telephones cannot proceed past step 59 and subsequent steps of FIG. 9 and cannot be used. Such a state may be caused by an EMI radiation generated by electronic devices and it is impossible to ensure a required quantity of traffic.
It is an object of this invention to provide a radio communication apparatus which ensures a large amount of traffic and is not influenced by cross talk and electric-wave interference.